When audio file spam first appeared it looked as if junk mailers were exploring the use of new attachment types in a bid to keep their junk mail output varied and shifting. PDF spam, for example, made its first appearance later in 2007 and has hung around ever since.

MP3 spam was more like a failed experiment, a demo tape that no-one cares to remember, so its reappearance two years on is a bit of a puzzler. Spammers have moved towards using a hyperlink that leads to spammers' websites more than any kind of attachment, much less a bulky audio file.

A recent MP3 spam run intercepted by MessageLabs features a voice-synthesized MP3 clip - just five seconds long - giving the name of a site flogging Viagra. Adding spice to the mix, spammers have added an imitation of Meg Ryan's infamous coffee-shop scene in When Harry Met Sally in the background.

The MP3 filenames are seemingly changed for each message... but the properties of the files reveal that the title of each MP3 recording is the same, "WWW.77557.NET - CHEAP VIAGRA".

The latest MP3 spam run features random characters in the lyrics tag, possibly so that each file is slightly different, in a possible attempt to bypass rudimentary signature based spam detection based on noting MD5 file hashes.

The unidentified spammers behind the MP3 run are not without a sense of humour. "The "Genre" of the MP3 recording is set to "Blues," presumably in a reference to the famous blue pill the spammers were touting," MessageLabs adds.

The MP3 spam run began on the afternoon of 16 December 2009 and ended by mid-morning on 17 December, it accounted for 1.2 per cent of all spam during its overnight peak. MessagLabs reckons as many as 500 million MP3 spam messages were distributed during the run.

"This could be the most frequently "downloaded" MP3 track in the world... whether its recipients want it or not," MessageLabs adds.

The MP3 spam was sent out from machines compromised by the "Climbot" botnet – estimated to be between 10,000 and 20,000 zombie PCs strong - a week after an image spam run, also touting penis pill sites. Climbot, whose zombie machines tend to be located in either continental Europe or South Korea, was last active as a source of spam back in June.

A write-up of the return of MP3 spam, containing audio clips (NSFW), can be found on MessageLabs website here. ®